How much of “Doctor Who” might really be possible?

Science shows why Doctor Who is so special.

As Doctor Who’s 50th anniversary looms, time travel is everywhere – on the screen, at least. Famously, the Doctor can whizz through the years using a “dimensionally transcendental” machine, the TARDIS, and make changes to the past as and when he likes. But what is time travel – and how much of “Doctor Who” might really be possible?

A handy definition of time travel comes from philosopher David Lewis. Lewis says time travel involves a journey having different durations viewed from outside (in “external time”) or from inside (in “personal time”). Suppose you spend five minutes travelling aboard your machine, as measured by (e.g.) your watch and your memories. On arrival, you find 150 years have elapsed in the outside world. Congratulations, you have time-travelled. Five minutes of your personal time has covered 150 years of external time.

Odd as this sounds, Einstein’s theory of Special Relativity introduced such possibilities to physics in 1905. The theory says: the duration of a process varies with the relative velocity of the observer. The closer that relative velocity gets to the speed of light, the longer the travelling process takes.

Suppose you want to see the Earth a billion years hence, but worry you have only about 50 personal years left. Special Relativity specifies that if you travel very close to the speed of light relative to the Earth, your 50 personal years can cover one billion Earth years.

In backward time travel, personal and external time differ in direction, so journeys end in external time before, not after, they begin; you spend five personal minutes travelling 150 years into the external past. General Relativity suggests that the universe is essentially curved spacetime, which might allow such divergences of external and personal time.

Relativity treats space and time as aspects of a single entity: “spacetime”. One of the more remarkable features of General Relativity is that it allows time and space axes to be interchanged, so one observer’s space axis can be another observer’s time axis.

In 1949, Austrian mathematician Kurt Gödel used General Relativity to describe a universe where intrepid voyagers can go anywhere in (past or future) time without travelling faster than light. Gödel’s universe has no boundaries in space or time, and all the matter in it rotates. But our finite, non-rotating universe is not Gödel’s. Despair not though – simply spin an ultradense, very (maybe infinitely) long cylinder very fast. Spacetime should curve around the cylinder so the direction of the local future partially points into the external past. Such devices are called “Tipler Cylinders”, after physicist Frank Tipler.

Better yet, quantum theory suggests that “wormhole” connections between different spacetime points spontaneously form and break all the time. The chances are that natural wormholes are tiny - vastly smaller even than an electron, (and a billion trillion electrons can fit in a teaspoon). But you could perhaps find (or create) a wormhole big enough and durable enough to let you slip through into the past. Difficult, but theoretically possible.

No, you can’t kill your physics teacher

So perhaps you could travel into the past. But what about paradoxes? What is to stop you assassinating your grandfather or yourself as infants? One answer says: logical consistency.

Classical logic says you cannot consistently kill in infancy someone who achieves adulthood. But, Lewis says, time travel need not involve doing the logically impossible – provided travellers’ actions in the past are consistent with the history whence they come. So you could try killing your baby grandfather, but something would foil you – you would sneeze, or your gun would jam. Lewisian time travel is therefore (classically) consistent, but might look very strange, since seemingly possible actions (like shooting an unprotected infant) would prove impossible.

Another view says that backward time travel requires many worlds – that is, many different but equally real versions of physical reality. Physicist David Deutsch and philosopher Michael Lockwood argue that time travel must involve inter-world travel. If you travel backwards in time, you must arrive in a history different from your native one and so would be quite unfettered by your past once you get there. You could even kill this other history’s counterparts of your grandfather and yourself.

Both these concepts of backwards time travel may disappoint anyone wanting to change the “one and only” past. Conventional logic says time travellers would either help make history what it was (Lewis) or create a different history (Deutsch/Lockwood). However, quantum logic might let travellers change the actual (one-and-only) past.

Suppose we hold that quantum measurements determine (or change) quantities measured, even if those quantities lie in the past. Someone could travel back and “observe” history turning out differently from how it originally was, thereby retrospectively making actuality different from what it had been. What would happen to travellers who rebooted history is not clear, but this model seems closer to the time travel familiar from “Doctor Who” and other fictions. Beware, though, because quantum theory allows no predicting, and still less controlling, of the outcomes of changing the past. There would be no way to foresee the effect you would have on the present.

So classical logic, General Relativity and quantum theory all seem to permit time travel. Classical logic plus General Relativity suggest backward travellers face weird consistency constraints. Many-worlds travellers face no constraints, but get displaced into different histories. Quantum-logic travellers could change the (one and only) past without constraints, but they couldn’t predict or control what they would get.

So far, however, it seems only the Doctor knows how to change the past at will.

Alasdair Richmond received funding from the Arts and Humanities Research Council, to fund one semester of a two-semester leave period that covered the academic year 2008-2009.

The Conversation

This article was originally published at The Conversation. Read the original article.

The eleven incarnations of the Doctor. Image: BBC/Matt Burlem

Alasdair Richmond is a Senior Lecturer in Philosophy at the University of Edinburgh.

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Sunjeev Sahota’s The Year of the Runaways: a subtle study of “economic migration”

Sahota’s Man Booker-shortlisted novel goes to places we would all rather not think about.

This summer’s crisis has reinforced the ­distinction that is often made between refugees, who deserve sanctuary because they are fleeing from conflict, and “economic migrants”, those coming to Europe in pursuit of “the good life”, who must be repelled at any cost. The entire bureaucratic and punitive capacity of our immigration system is pitted against these ne’er-do-wells and their impudent aspirations.

Sunjeev Sahota’s fine second novel, The Year of the Runaways, now shortlisted for the Man Booker Prize, takes a closer look at “economic migration”. Why do people – many of them educated, from loving families in peaceful communities – leave their old lives behind and come to Britain? Are they fleeing desperate circumstances or are they on the make? When they arrive here, do they find what they were looking for? Should we welcome them, or try to persuade them to stay at home? The book illuminates all of these questions while, much to its credit, offering no simple answers.

Sahota interweaves the stories of three people whose reasons for emigrating are as individual as they are. Both Avtar and Randeep are from Indian Sikh families that might be characterised as lower-middle-class. Avtar’s father has his own small business – a shawl shop – and Randeep’s father works for the government. Both boys are educated and Avtar, in particular, is smart and motivated. But with employment hard to come by and no social security net to fall back on, it doesn’t take much to make leaving the country seem like the only option. Avtar loses his job, his father’s business is failing and he has high hopes of earning enough to marry Lakhpreet, his girlfriend-on-the-sly. Randeep’s family’s finances fall apart after his father has a psychological breakdown; their only hope of maintaining a respectable lifestyle is for their eldest son to take his chances abroad.

For Tochi, the situation is very different. He is what used to be called an “untouchable” and, although people now use euphemisms (“scheduled”, or chamaar), the taboo remains as strong as ever. He comes to Britain not so much for financial reasons – although he is the poorest of the lot – but to escape the prejudice that killed his father, mother and pregnant sister.

Tying these disparate stories together is the book’s most intriguing character, Narinder, a British Sikh woman who comes to believe that it is her spiritual calling to rescue a desperate Indian by “visa marriage”. Narinder’s progress, from the very limited horizons for an obedient young woman to a greater sense of herself as an active participant in her destiny, reminded me of Nazneen, the protagonist in Monica Ali’s Brick Lane. But Narinder is a more thoughtful character and here the Hollywood-style journey of personal liberation is tempered by a recognition of the powerful bonds of tradition and family.

Once in Britain, Avtar, Randeep and Tochi enter a world of gangmasters, slum accommodation and zero job security, with an ever-present fear of “raids” by immigration officers. They work in fried chicken shops, down sewers, on building sites and cleaning nightclubs. Health care is off-limits for fear of immigration checks. Food is basic and the only charity comes from the gurdwara, or Sikh temple, which provides help in emergencies.

Avtar and Randeep struggle to send money back home while living in poverty and squalor that their families could barely imagine (at one point, Randeep notes with understandable bitterness that his mother has used his hard-earned contributions to buy herself a string of pearls). In the meantime, their desperation leads them to increasingly morally repellent behaviour, from selfishness to stealing and worse. Even if they do eventually find a measure of economic stability in Britain, they have done so at the cost of their better selves.

It has been pointed out that the novels on the Man Booker shortlist this year are even more depressing than usual and The Year of the Runaways certainly won’t have raised the laugh count. At times I had to put it down for a while, overwhelmed by tragedy after tragedy. It was the quality of Sahota’s prose and perceptions that brought me back. He is a wonderfully subtle writer who makes what he leaves unsaid as important as the words on the page. A wise and compassionate observer of humanity, he has gone to some dark places – places we would all rather not think about – to bring us this book. Whether we are prepared to extend a measure of his wisdom and compassion to real immigrants, in the real world, is another question.

“The Year of the Runaways” by Sunjeev Sahota is published by Picador (480pp, £14.99)

Alice O'Keeffe is an award-winning journalist and former arts editor of the New Statesman. She now works as a freelance writer and looks after two young children. You can find her on Twitter as @AliceOKeeffe.

This article first appeared in the 08 October 2015 issue of the New Statesman, Putin vs Isis